This invention relates to a flexible circuit or wiring board and a method of making the same. More particularly, this invention relates to a flexible circuit which incorporates terminal or termination features (i.e., termination or terminal-like pins) as an integral part thereof thereby permitting connection between flexible circuits and other circuits or components without the use of an intermediate connector device therebetween.
Interconnections between flexible circuit boards and other circuit boards or components have been a source of problems for many years. A conventional method of effecting such interconnections has been to use separate connecting devices to link circuit traces on the flexible circuit to the mating circuitry. Typically, these connecting devices are comprised of well known high density pin and socket connectors which must be soldered, crimped or otherwise attached to circuit traces on the flexible circuit. However, it will be apparent that drawbacks and deficiencies are associated with the use of such separate connectors. These drawbacks include higher costs in terms of manufacturing and purchasing, higher weight and size (associated with the connectors), inability to achieve desired density of interconnections, and increased chance for failure because of the multiplicity of connections which must be made between each contact in the connector and the traces on the flexible circuit.
Recently, an alternative solution to providing electrical connections between flexible circuits and other electrical devices which eliminates the use of separate electrical connectors has been suggested. In this alternative technique, a flexible circuit comprising a plurality of spaced metallic conductors include one or more flexible areas and integral rigid terminals. These terminals are formed from a relatively rigid metallic sheet by selectively reducing the sheet in cross-section so as to define the conductor patterns and terminal ends and to render flexible, (at areas of reduced cross-section), areas of the conductors. The metallic flexible conductors and rigid terminal ends are then attached to flexible insulating films to support the conductors and to maintain them in spaced relation to one another; or alternatively the metallic conductor is predisposed as a laminate on a film. The metallic sheet preferably initially has a thickness substantially equal to that required for the terminal ends. Alternatively, it is also possible to start with a metallic sheet, somewhat thinner than that required for the terminal ends, process as before, and then plate additional metal onto the terminal ends to achieve a required thickness.
In still another alternative embodiment, a metallic sheet having a thickness approximately equal to the conductor flexible areas is used as a starter material. Next, one or more mesas are formed on the edge regions of the sheet so that the thickened edge regions are integral with the sheet central areas. Thereafter, the conductive areas are chemically milled so as to define conductor patterns and terminal areas. This latter process is described in U.S. Pat. No. 4,085,502.
Flexible circuits of the type described above (which are characterized by thinning down portions of a metal sheet to form flexible traces and rigid terminal pins) are marketed under the trademark SCULPTURED by Advanced Circuit Technology of Merrimack, N.H.
While solving some of the disadvantages associated with separate connector devices, the SCULPTURED type of flexible circuit has many of its own problems. For example, the manufacturing technique utilizes conductive material (i.e., copper) two to three times thicker than necessary with most of this copper being etched away in at least two imaging and etching steps so that the central portion is thinned down and the edges remain thick and rigid for connection to another circuit component. As a result, the SCULPTURED circuits are quite expensive to manufacture due to the costly substraction (etching) processing steps and increased materials cost. Furthermore, due to the imprecision of this extensive etching process conductor definition is frequently not accurate enough to meet demanding technical requirements for certain electrical properties, e.g. controlled impedance. Also, this method may create conductors that would frequently be unsuitable for dynamic applications.